After being on a supplement for three months, I remove it from my daily intake for a week to see if anything happens. It can take a long time for supplements to start working, but I've found that their effects taper off quick when I stop, so a week is usually long enough to see if there's merit or not (even with slow-acting things like Vitamin D).
So that's what I'm doing with B5 and l-carnitine right now. The first two weeks of being on l-carnitine were miserable, and then I got sick so I never could tell if there was improvement or if the way I was feeling was tied to the supplement.
It's kind of expensive so I'm sort of hoping I don't feel any different after a week, but at the same time I would prefer it if it did have a noticeable impact. It would mean I'm on the right track.